1. Field of the Invention
This invention relates to dielectric films for capacitors and the like comprising vinylidene fluoride polymer, having improved electrical and optical properties. This invention also relates to processes for producing such films.
2. Brief Description of the Prior Art
Films comprising vinylidene fluoride polymer have high dielectric constants and permit small-sized capacitors to be formed using the films as a dielectric material. Ordinary melt-extruded films, cast films prepared through solvent evaporation, and pressed films have dielectric constants (or relative permittivities) of up to only about 9-10 at room temperature and at a frequency of about 1 KHz however. Higher dielectric constants are desired. Moreover, the dielectric loss, tan .delta., at room temperature and at a low frequency region below commercial frequencies is typically more than about 3-5%.
To increase the dielectric constant and reduce tan .delta. of films of vinylidene fluoride polymers, an improved process of stretching formed, unoriented or unstretched film at least in one direction and of setting or fixing it has been developed heretofore and put to practical use (see Japanese Patent Publication No. 17680/1975). The film prepared by this method has an improved tan .delta. of less than about 3% at low frequency regions and improved dielectric constants, higher than about 10-11 at room temperature and at about 1 KHz, as compared with that of unstretched film. However, the stretched and oriented film has a tendency to shrink upon heating. While shrinkage can be avoided by heat treating the stretched and oriented film at temperatures approaching the crystalline melting point of the film, the molecular orientation of the resin in the film may be varied thereby with concommitant changes in the physical properties of the film. Accordingly, it has not been possible to obtain a capacitor making use of stretched films of vinylidene fluoride polymers free from heat shrinkage at temperatures up to about the crystalline melting point of the polymer. Thus, films of vinylidene fluoride polymer having improved dielectric properties such as increased dielectric constants and lower tan .delta., obtained by stretching operations, were liable to experience significant and irreversible changes in dielectric properties owing to shrinkage.
Further, in some cases, the dielectric constant of the stretched and oriented vinylidene fluoride polymer film as measured between commercial frequencies and 1 KHz remarkably increases as the temperature rises from about 0.degree. C. to about 80.degree. C. For instance, it indeed shows an increase of greater than about 10% between 20.degree. C.-80.degree. C., and greater than about 30% between 0.degree. C.-80.degree. C. If the dielectric constant of the dielectric material in a capacitor changes, this causes the change in electrostatic capacitance and, further, various properties of a circuit using this capacitor may vary. Furthermore, if an irreversible change occurs due to heat shrinkage or the like, it becomes impossible to forecast the temperature-dependent change in the electrical constant of the circuit, rendering devices made thereby useless.
It is known that the stretching operation for improving the dielectric property of films of vinylidene fluoride polymer causes the conversion of crystal form between the two main crystal structures of the vinylidene fluoride polymer, that is, from alpha-form ("II-phase") to highly polar beta-form ("I-phase"). It is also known that, when exposed to a high electric field, vinylidene fluoride polymer of the beta-form exhibits high piezoelectric and pyroelectric properties. For example, when vinylidene fluoride polymer film of the beta-form is used for a surge absorber in an electronic circuit, if it is subjected to a dielectric strength test under the application of a high voltage before mounting or exposed to a considerably high voltage which is lower than the dielectric strength after mounting, the piezoelectric and pyroelectric properties are given to the film. Since undesired phenomena such as generation of noise signals may thus be caused by changes in the ambient temperature, the provision of such piezoelectric and pyroelectric properties is usually to be avoided in application of the film for capacitors.
In addition, most of the unstretched or unoriented vinylidene fluoride polymer films contain spherulites, and the oriented films prepared by stretching these spherulites often have uneven surfaces and poor optical tranparencies. It is thus desired to eliminate such unevenness, for improving the characteristic properties such as dielectric strength required for electrical materials such as capacitors.
The present inventors have discovered methods to prepare dielectric films from vinylidene fluoride polymers having improved dielectric constants and improved tan .delta. without the foregoing undesirable properties.